Surge in fuel prices and introduction of environmental regulations in recent years have created the demand for highly fuel-efficient tires, and there is a need that the tread, which largely contributes to fuel economy, be more fuel efficient. Since most treads contain natural rubber, the fuel economy of natural rubber also needs to be improved to improve the entire tire.
Patent Literature 1, for example, discloses modification of natural rubber to improve fuel economy, in which natural rubber latex is combined with a surfactant and washed. However, this method can reduce the protein and gel contents to some extent, but not to desired levels, and further reduction in tan δ is desired. Moreover, heat aging resistance and other properties are also required for rubber for tires. However, the method of Patent Literature 1 cannot provide sufficient heat resistance and there is a need for improvement to simultaneously ensure fuel economy and heat aging resistance.
Natural rubber has a higher Mooney viscosity than other synthetic rubbers and is poor in processability. Usually, natural rubber is mixed with a peptizer and masticated to reduce the Mooney viscosity before use, which results in poor productivity. Further, since the mastication breaks the molecular chains of natural rubber, the resultant natural rubber unfortunately loses the high-molecular-weight polymer characteristics that natural rubber originally has, such as good fuel economy, abrasion resistance, and rubber strength.
Meanwhile, many tires these days have a two-layer structure consisting of a base tread and a cap tread. The cap tread, which is a component directly contacting the road surface, is expected to have properties to match various environments as well as abrasion resistance. For use in summer tires it is expected to have particularly wet-grip performance, while for use in studless winter tires it is expected to have particularly grip performance such as performance on snow and ice. Moreover, along with improvements in the performance of automobiles and the development of road networks, the base tread, which forms an inner component, also needs to impart improved handling stability, in particular during high speed driving, to tires.
As described above, rubber compositions for tires are desired which achieve further improvements in properties such as fuel economy, processability, heat aging resistance, abrasion resistance, wet-grip performance, performance on snow and ice, and handling stability.